A digital video stream may be partitioned into successive groups of pictures (GOPs), where each picture or frame in the GOP may be of a pre-defined picture coding type. Such digital video streams may conform to the Moving Pictures Experts Group (MPEG)-2 standard, the MPEG-4 advanced video coding (AVC)/H.264 standard, the emerging high efficiency video coding (HEVC) standard, and/or other formats. The picture coding types may comprise intra-coded pictures, unidirectional-predicted pictures, and bidirectional-predicted pictures. The intra-coded pictures, or I-pictures, may only use the information within the picture to perform video compression or encoding. These self-contained I-pictures provide a base value or anchor that is a prediction of the value of succeeding pictures. Each GOP may generally start with a self-contained I-picture as the reference or anchor frame from which the other frames in the group may be generated for display by a decoder or for reconstruction by an encoder. A GOP may start with an I-picture when describing the transmission, decoding, and/or processing order of a picture sequence, for example.
The GOP frequency, and correspondingly the frequency or periodicity of I-pictures, may be driven by specific application spaces. The predicted, or P-pictures, may use a motion estimation scheme that generates motion vectors that may be utilized to predict picture elements from previously encoded pictures. Compressing or encoding the difference between predicted samples and the source value results in better coding efficiency than that which may be achieved by transmitting the encoded version of the source picture information. At a receiver or decoder side, the compressed or encoded difference picture is decoded and subsequently added to a predicted picture for display.
The bidirectional-predicted pictures, or B-pictures, may use multiple pictures that occur in a future location in the GOP and/or in a past location in the GOP to predict the image samples. As with P-pictures, motion estimation may be used for pixel prediction in B-pictures and the difference between the original source and the predicted pictures may be compressed. At the receiver, or decoder, end, one or more B-pictures may be motion compensated and may be added to the decoded version of the compressed difference signal for display. Since both the P-pictures and B-pictures may be based on other pictures, they may be referred to as inter-coded pictures.
In the AVC standard, instead of B-picture, P-picture, and I-picture types, the type definitions are made slice-wise, where a slice may cover an entire picture. However, it is commonly accepted practice to refer to I-pictures as pictures which only contain intra-coded or I-slices, P-pictures as pictures which may contain predicted slices (P-slices) or I-slices, and B-pictures as pictures which may contain bi-directional or bi-predictive slices (B-slices), P-slices, or I-slices. In the AVC standard, the B-pictures may also be used for prediction, further complicating the decoding dependency between pictures in a bit stream.